In order to increase the power reserve, by increasing the number of turns of a mainspring, secured at the inner end thereof to a core formed either by an arbor, which is generally cylindrical, or by a more solid boss, one solution consists in decreasing the diameter of the barrel arbor and of the associated boss, so as to increase the space available for the spring inside the drum.
The ratio of the core radius to the spring thickness is usually comprised between 10 and 20 and the invention proposes to reduce this ratio to below 10, and preferably to within a range of between 5 and 10.
The sizing must not be too small; there is a risk of breakage if the diameter of the core is too small.
In the conventional barrel architecture, a ratchet is axially mounted on a barrel arbor or on a core, via a square, with the ratchet usually being secured by an axial screw. The dimension of this screw and that of the square thus define the minimum diameter of a pivot shoulder. A step portion joined to this pivot shoulder limits the endshake of the arbor or of the core relative to a bottom plate or to a bridge carrying a jewel or similar element.
In particular, it is not sufficient merely to reduce all of the dimensions, since the cross-sections of material are then insufficient to ensure fatigue resistance.
The issue is thus to reconcile the smallest possible diameter, to allow the largest possible power reserve, with rigidity in the ratchet drive.
Thus, it is necessary to devise a barrel with a ratchet and arbor such that the smallest possible arbor diameter can pass the torque transmitted by the ratchet.
FR document No 2329000A1 in the name of ETA discloses a barrel with a non-cylindrical arbor having a drum element, the hook hooking the spring being contained in a recess delimited by a geometric cylinder defined by the upper and lower shoulders of the drum element.
XP document No 001219052A in the name of SOCIETE ODO discloses a barrel with a non-cylindrical arbor, carrying a tube which in turn carries the hook for hooking the spring. This arbor includes shoulders and grooves for circlips.
US document No 730103A in the name of ERICKSON discloses a non-cylindrical arbor including a faceted portion, called a rectangular portion, which carries a core bearing the hook for hooking the spring
FR document No 2135134A1 in the name of BAUMGARTNER discloses a barrel with a slit sleeve carrying a barrel core.
CH document No 15286A in the name of DANASINO discloses a barrel with a spring whose cross-section gradually increases from the exterior to the interior of the drum.
FR document No 1443494A in the name of GLASHUETTER UHRENBETRIEB discloses a barrel with a cover plate which limits the axial travel of the barrel arbor.
FR document No 2210784A1 in the name of ETA discloses a single-piece arbor including a longitudinal groove for hooking the spring.